Sort label providers to make labeling deterministic

src/core/pal/pal.h

@@ -37,13 +37,48 @@
 #include "qgsgeometry.h"
 #include "qgsgeos.h"
 #include "qgssettingstree.h"
+#include "qgslabelingengine.h"
 
 #include <QList>
 #include <iostream>
 #include <ctime>
 #include <QMutex>
 #include <QStringList>
-#include <unordered_map>
+
+struct CompareLabelProvider
+{
+  bool operator()( const QgsAbstractLabelProvider *lhs, const QgsAbstractLabelProvider *rhs ) const
+  {
+    if ( !lhs )
+    {
+      return true;
+    }
+    if ( !rhs )
+    {
+      return false;
+    }
+
+    QString lhsProviderId = lhs->providerId();
+    QString rhsProviderId = rhs->providerId();
+
+    if ( !lhsProviderId.isEmpty() && !rhsProviderId.isEmpty() )
+    {
+      return lhsProviderId < rhsProviderId;
+    }
+    else if ( !lhsProviderId.isEmpty() )
+    {
+      return true;
+    }
+    else if ( !rhsProviderId.isEmpty() )
+    {
+      return false;
+    }
+    else
+    {
+      return lhs->layerId() < rhs->layerId();
+    }
+  }
+};
 
 class QgsSettingsEntryInteger;
 
@@ -279,7 +314,7 @@ namespace pal
 
     private:
 
-      std::unordered_map< QgsAbstractLabelProvider *, std::unique_ptr< Layer > > mLayers;
+      std::map< QgsAbstractLabelProvider *, std::unique_ptr< Layer >, CompareLabelProvider > mLayers;
 
       QList< QgsAbstractLabelingEngineRule * > mRules;